Sheeting into which a visible pattern or legend is built has found a number of important uses, particularly as distinctive labels useful to authenticate an article or document. Such sheetings, as taught in U.S. Pat. No. 3,154,872 (Nordgren); U.S. Pat. No. 3,801,183 (Sevelin et al.); U.S. Pat. No. 4,082,426 (Brown); and U.S. Pat. No. 4,099,838 (Cook et al.), have found use as validation stickers for vehicle license plates and as security films for driver's licenses, government documents, phonograph records, tape cassettes, and the like. These references teach that the legends must be incorporated into the sheeting when the sheeting is being made.
U.S. Pat. No. 4,200,875 (Galanos) discloses a method of forming directional images in retroreflective sheeting which comprises a monolayer of glass microspheres and a reflective masking layer. In that method, laser irradiation of the retroreflective sheeting in an imagewise fashion causes structural alterations or modifications in the sheet which are viewable as directional images.
Hockert et al., Jap. Appln. No. 19824/84, filed Feb. 6, 1984, Kokai No. 148004/84, laid open Aug. 24, 1984, forms a directional image in sheeting which comprises a monolayer of microlenses, a masking layer which may be a reflective material or a thin laser penetrable material disposed behind the monolayer of microlenses, and a transparent spacing layer located between those two layers. The sheeting taught therein may also employ one or more transparent layers on the opposite side of the monolayer of microlenses from the spacer layer. Sheeting with such additional layer(s) is known as "enclosed-lens" or "embedded-lens" retroreflective sheeting.
The latter reference teaches the irradiation of such sheeting with a laser beam to form openings, or axial markings, in the masking layer which are visible as a directional image. The transparent spacer layer may be colored at the axial markings to provide a colored directional image. Further, when the axial markings have been colored, the masking layer may be removed, thereby making the sheet transparent. The sheet may then be adhered to a document as an overlay which provides a secure method of authenticating the document.